viernes, 29 de diciembre de 2017

Button Panel from Old TV Set

A TV Selector Board
I have found a selector board from an "old" TV set. The main parts were already taken away, and I do not even know the brand.
The board is rectangular in its general shape, with dimensions 19*5 cm, approx. It is connected through a 7 wire bus.

Beko Elektronik
The board is labelled 7V2.191 in the back, and "ZK4 172 - 40800386" in a code bar sticker. Under it: "7V2.191 Beko Elektronik".

There is a "" specialized in HF amplifiers, which I guess is probably not the same. There are also some webs in internet with "Beko Elektronik" spare parts: one, another, ebay, Google images.

The button board has four main areas: the set of 4 push buttons, an IR receiver, a LED, and the 7-wire bus.
The schematics is as follows:
The values of the passive components are:
- R901: 150 ohm
- R902: 150 ohm
- R903: 1500 ohm
- R904: 220 ohm
- C901: 10 uF, 16 V

JC901 is an infrared receiver module with a PIN diode and a preamplifier. Marks are: "7RZ 230" on the socket, and " 19 427" and "V34838T" (perhaps B instead of 8) on the receiver. I have not found information on it, but it is probably similar to this.

A common pin convention for IR receivers seems to be:
- 1: Vout
- 2: GND
- 3: Vcc
It is unclear this applies here: pin 1 is labelled on the board as the one on the right, while in all datasheets I have seen, pin 1 is on the left, and it is Vout:

The recommended external components are shown in some datasheets:

In the board, pin 2 is clearly ground, and the electrolytic capacitor is connected to pin 1 (on the right) and ground, so this "pin 1" seems to be Vcc, or the "pin 3" of the datasheets.
(Of course it is easy to test where is Vout, and to avoid the doubts).

Some datasheets make reference to the frequency of the receiver. "Universal" seems to be 38 kHz, at least for common home apparel, but not all receivers seem to operate at this frequency:
(I have also not checked this).

So, the operation seems to be as follows:
- On the main header (JX901) pin 2 is ground, and pin 4 Vcc (unknown voltage, likely in the range of 5 V). Whenever pin 4 is on, LED L1 is on. If the path through R904 is open (pin 3 in the main header), then L1 is lighting bright; if this path is not allowed, then L1 closes its circuit through R903, dimmed.
- The signal from the IR receiver is sent through pin 1 of the main header (surprisingly, through R901).
- The push buttons send signals with different voltage levels:
* V-: pin 6 to pin 7
* V+: pin 5 to pin 7
* P-: pin 5 to pin 7
* P+: pin 7 to ground.

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